1. Field of the Invention
This invention relates to a steering apparatus for steering steerable wheels in a wheeled vehicle and, more particularly, to a steering apparatus capable of decreasing the steering gear ratio in accordance with increase of the rational angle of the steering wheel.
2. Prior Art of the Invention
In general, a conventional front wheel steering apparatus of this kind includes an input shaft arranged to be rotated by the driver's steering effort applied thereto, an output member operatively connected to a set of steerable front wheels by means of a steering linkage, and an interconnecting mechanism in the form of a rack and pinion mechanism, a ball screw mechanism or the like for operatively interconnecting the input shaft to the output member. In the interconnecting mechanism, however, a displacement ratio of the output member relative to a rotational amount of the steering wheel is constant. For this reason, the driver's feel in steering operation becomes insufficient, i.e., the driver feels that the steering operation is too light, when the steering wheel is steered at a large angle from a neutral position, and a maximum rotational angle of the steering wheel necessary for steering the front wheels at a desired maximum angle becomes large (for example, 540 degrees). In the case that the displacement ratio of the output member is increased to overcome the above problem, the driver's feel in steering operation becomes excessive, i.e., the driver feels that the steering operation is too heavy, at an initial stage of steering operation from the neutral position of the steering wheel.
A front steering apparatus disclosed in Japanese patent laid open No. 14971/1990 has been proposed to overcome the above-mentioned problem. In this steering apparatus, an intermediate shaft is arranged in parallel relationship with the coaxially arranged input and output shafts of the steering apparatus. A pair of oval gears are fixed to the input shaft and intermediate shaft, respectively, for coupling the input shaft and intermediate shaft through the oval gears while a pair of circular gears are fixed to the intermediate shaft and the output shaft, respectively, for coupling the intermediate shaft and the output shaft. With this construction, the displacement ratio of the output shaft to the input shaft increases in accordance with increase of rotational angle of the input shaft from the neutral position thereof. This steering apparatus is also provided with a hydraulic power cylinder for applying an assist force to the output operation member connected to steerable wheels, and a servo valve which responds to an input torque applied to the input shaft to supply a pressurized fluid to a pair of cylinder chambers of the power cylinder.
The steering apparatus, however, has a problem that the machining of the pair of the oval gears is difficult, and that a desired steering feel cannot be obtained, because the steering feel changes depend on the meshing condition of the gears. Further, since it is difficult to simultaneously reduce a backlash between the pair of the circular gears and a backlash between the pair of the oval gears, the assembly of the steering apparatus is troublesome.